Multiplexing multiple links on 1 physical line common on long-haul, high capacity links have FDM, TDM, CDM and WDM

Frequency Division Multiplexing (FDM)

FDM System Overview

FDM Voiceband Example

Analog Carrier Systems long-distance links use an FDM hierarchy AT&T (USA) and ITU-T (International) variants Group –12 voice channels (4kHz each) = 48kHz –range 60kHz to 108kHz Supergroup –FDM of 5 group signals supports 60 channels –carriers between 420kHz and 612 kHz Mastergroup –FDM of 10 supergroups supports 600 channels original signal can be modulated many times

North American and International FDM Carrier Standards

Wavelength Division Multiplexing (WDM) multiple beams of light at different frequenciescarried over optical fiber links commercial systems with 160 channels of 10 Gbps lab demo of 256 channels 39.8 Gbps architecture similar to other FDM systems multiplexer consolidates laser sources (1550nm) for transmission over single fiber optical amplifiers amplify all wavelengths demultiplexer separates channels at destination Dense Wavelength Division Multiplexing (DWDM) use of more channels more closely spaced

ITU WDM Channel Spacing (G.692)

Synchronous Time Division Multiplexing

TDM System Overview

TDM Link Control no headers and trailers data link control protocols not needed flow control –data rate of multiplexed line is fixed –if one channel receiver can not receive data, the others must carry on –corresponding source must be quenched –leaving empty slots error control –errors detected & handled on individual channel

Data Link Control on TDM

Framing no flag or SYNC chars bracketing TDM frames must still provide synchronizing mechanism between source and destination clocks added digit framing is most common –one control bit added to each TDM frame –identifiable bit pattern used as control channel –alternating pattern …unlikely to be sustained on a data channel –receivers compare incoming bits of frame position to the expected pattern

Pulse Stuffing Pulse Stuffing is a common solution have outgoing data rate (excluding framing bits) higher than sum of incoming rates stuff extra dummy bits or pulses into each incoming signal until it matches local clock stuffed pulses inserted at fixed locations in frame and removed at demultiplexer - problem of synchronizing various data sources - variation among clocks could cause loss of synchronization - issue of data rates from different sources not related by a simple rational number

TDM Example

Digital Carrier Systems long-distance links use a TDM hierarchy AT&T (USA) and ITU-T (International) variants US system based on DS-1 format can carry mixed voice and data signals 24 channels used for total data rate 1.544Mbps each voice channel contains one word of digitized data (PCM, 8000 samples per sec) same format for 56kbps digital data can interleave DS-1 channels for higher rates DS-2 is four DS-1 at 6.312Mbps

North American and International TDM Carrier Standards

DS-1 Transmission Format

SONET/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) high speed capability of optical fiber defines hierarchy of signal rates Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) is 51.84Mbps carries one DS-3 or multiple (DS1 DS1C DS2) plus ITU-T rates (e.g., 2.048Mbps) multiple STS-1 combine into STS-N signal ITU-T lowest rate is Mbps (STM-1)

SONET/SDH Signal Hierarchy

SONET Frame Format

Statistical TDM in Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand multiplexer scans input lines and collects data until frame is full line data rate lower than aggregate input line rates may have problems during peak periods must buffer inputs

Statistical TDM Frame Format

Single-Server Queues with Constant Service Times and Poisson (Random) Arrivals

Cable Modems Downstream cable scheduler delivers data in small packets active subscribers share downstream capacity also allocates upstream time slots to subscribers Upstream user requests timeslots on shared upstream channel headend scheduler notifies subscriber of slots to use -dedicate two cable TV channels to data transfer -each channel shared by number of subscribers using statistical TDM

Cable Spectrum Division to support both cable television programming and data channels, the cable spectrum is divided in to three ranges: – user-to-network data (upstream): MHz – television delivery (downstream): MHz – network to user data (downstream): MHz

Cable Modem Scheme

Asymmetrical Digital Subscriber Line (ADSL) link between subscriber and network uses currently installed twisted pair cable is Asymmetric - bigger downstream than up uses Frequency Division Multiplexing –reserve lowest 25kHz for voice (POTS) –uses echo cancellation or FDM to give two bands has a range of up to 5.5km

ADSL Channel Configuration

Discrete Multitone (DMT) multiple carrier signals at different frequencies divide into 4kHz subchannels test and use subchannels with better SNR 256 downstream subchannels at 4kHz (60kbps) –in theory 15.36Mbps, in practice 1.5-9Mbps

DMT Transmitter

Broadband – Customer Side DSL link is between provider and customer a splitter allows simultaneous telephone and data service data services use a DSL modem –sometimes referred to as G.DMT modem DSL data signal can be divided into a video stream and a data stream –the data stream connects the modem to a router which enables a customer to support a wireless local area network

Broadband – Provider Side a splitter separates telephone from Internet voice traffic is connected to public switched telephone network (PSTN) data traffic connects to a DSL multiplexer (DSLAM) which multiplexes multiple customer DSL connections to a single high- speed ATM line. ATM line connects ATM switches to a router which provides entry to the Internet

xDSL high data rate DSL (HDSL) –2B1Q coding on dual twisted pairs –up to 2Mbps over 3.7km single line DSL –2B1Q coding on single twisted pair (residential) with echo cancelling –up to 2Mbps over 3.7km very high data rate DSL –DMT/QAM for very high data rates –separate bands for separate services

Comparison of xDSL Alternatives

Summary multiplexing multiple channels on single link FDM –a–analog carrier systems –w–wavelength division multiplexing TDM –T–TDM link control –p–pulse stuffing statistical TDM broadband ADSL and xDSL